The disclosed invention relates to methods and apparatus for communicating with spaced apart electronic units, and more particularly to methods and apparatus for communicating underwater with acoustic transponders. Such transponders are utilized for example to determine the distance or velocity of ships with respect to the transponder. Basically, distance measurements are made by sending a signal to the transponder, receiving a reply signal from the transponder, and multiplying the response time by the velocity at which the acoustic signals travel. Ship velocity is basically determined by transmitting signals of one frequency from the ship, reflecting these signals from the transponder, receiving the reflected signals in the ship, and examining the received signals for a doppler frequency shift.
Prior art transponders which are utilized to perform these functions are basically of two types. One type transponder simply operates a repeater station. That is, the signals received at the transponder are amplified and retransmitted without other processing or delay. However, a problem associated with such repeater transponders is that their transmit section and receive section must be physically separated; otherwise they will form a closed loop and go into oscillation. The amount of required separation and actual separation are variables which are difficult to determine. Thus, these variables detract from the accuracy with which distance and velocity measurements can be made.
Conventional transponders of a second type have the transmit and receive sections physically integrated in one unit. In operation, these transponders listen for a "triggering" or "recognition" signal. When the triggering signal is received, the transponder internally generates and transmits a pre-programmed response signal. This transponder avoids closed loop osillations because the response signal will not trigger the receive section. However, this transponder is deficient in that the time it takes to recognize the triggering signal is not accurately predictable. For example, the recognition time is dependent upon the signal level received. Thus, the time delay between receipt of a triggering signal and transmission of a response signal is a variable, which detracts from the accuracy of the distance and velocity measurements which can be made. Further, with this type transponder, the response signal is fixed, or is at least limited, to some preprogrammed number in accordance with the complexity of the transponder. Thus, the flexibility of the transponder is limited in that its responses cannot be readily varied.
Accordingly, it is one object of the invention to provide an improved transponder.
Another object of the invention is to provide a transponder having a precise predetermined response time.
Another object of the invention is to provide a transponder wherein signals are received and signals representative of the received signals are retransmitted after a precise time delay.
Another object of the invention is to provide an improved method for communicating with a remote transponder.
Another object is to provide a method of determining, to a high degree of precision, distance from a remote transponder.
Still another object of the invention is to provide a method of determining, to a high degree of precision, velocity with respect to a remote transponder.